Tm01 mode exciter and a multimode exciter using same

ABSTRACT

Dual elongated apertures are cut along a broad wall of a rectangular waveguide to excite in electromagnetic waves the TM01 mode in a circular waveguide symmetrically joined to the dual apertured region. A multimode exciter system is provided wherein the TM01 mode wave is excited as described above and a linearly polarized TE11 mode wave is excited in the circular waveguide by a second rectangular waveguide section coupled along the side walls of the circular waveguide. A second TE11 mode wave oriented orthogonal to the first mentioned TE11 mode wave is excited in the circular waveguide by an elongated aperture cut along the center of the broad wall of the first-mentioned rectangular waveguide opposite that of said first-mentioned broad wall with a third rectangular waveguide symmetrically joined to the center apertured region. A dual channel rotary joint system is herein described using two such multimode exciter systems with the circular waveguide sections connected to each other by a rotary joint section.

United States Patent 11 1 Woodward [54] TM MODE EXCITER AND A MULTIMODEEXCITER USING SAME [75] Inventor: Oakley McDonald Woodward, Princeton,NJ.

[73] Assignee: RCA Corporation, New York, NY. [22] Filed: Sept. 4, 1970[2]] Appl. N0.: 69,842

[11] 3,715,688 1 Feb. 6, 1973 Primary Examiner-Paul L. GenslerAttorney-Edward J. Norton [57] ABSTRACT Dual elongated apertures are outalong a broad wall of a rectangular waveguide to excite inelectromagnetic waves the TM mode in a circular waveguide symmetricallyjoined to the dual apertured region. A multimode exciter system isprovided wherein the TM mode wave is excited as described above and alinearly polarized TE, mode wave is excited in the circular waveguide bya second rectangular waveguide section coupled along the side walls ofthe circular waveguide. A second TE mode wave oriented orthogonal to thefirst mentioned TE, mode wave is excited in the circular waveguide by anelongated aperture cut along the center of the broad wall of thefirst-mentioned rectangular waveguide opposite that of saidfirst-mentioned broad wall with a third rectangular waveguidesymmetrically joined to the center apertured region. A dual channelrotary joint system is herein described using two such multimode excitersystems with the circular waveguide sections connected to each other bya rotary joint section.

14 Claims, 3 Drawing Figures TMm MODE EXCITER AND A MULTIMODE EXCITERUSING SAME This invention herein described was made in the course of orunder the contract or subcontract thereunder with the Department of theAir Force.

This invention relates to mode exciters and more particularly to a TMmode exciter.

A TM mode electromagnetic wave in circular waveguide is characterized bya radially extending electric field with constant amplitude and phase asa function of angular rotation about the periphery. This characteristicof the TM mode wave makes it ideal for use with rotary joint systems.This TM mode wave is transferred unaltered across a rotating section ofa circular waveguide rotating relative to another section of circularwaveguide without presenting distortion or discontinuity between thecircular waveguides.

One means of exciting the TM mode wave in circular waveguide is by meansof a coaxial probe fed from a TEM coaxial transmission line. Theimpedance bandwidth of this means for exciting a TM mode wave has beenfound to be rather narrow.

Another scheme developed to excite a TM mode wave is that of a rightangle junction of a rectangular waveguide to a circular waveguide with astep transition at the interface so that a wave inthe dominant TE modeis suppressed. Although this approach has a greater impedance bandwidththan that obtained by the coaxial probe, suppression of the dominant TEmode wave in circular waveguide is difficult. The addition of adiametrical conductive vane can aid in the suppression of the TE, modewaves at the expense of greater height of the structure in the directionof propagation. This suppression ofthe dominant TE mode wave is anecessity when the dominant TE mode wave is carrying different signalinformation and must be isolated from the TM mode wave. It is thereforedesirable to provide a TM, mode exciter which has relatively wideimpedance bandwidth and one in which only an electromagnetic wave in theTM mode is excited and not waves in the dominant TE mode. There may alsobe a requirement in many applications for either a single exciter or amultimode exciter that is compact.

In some communication applications such as when using space satellites,the antenna scans the skies in azimuth and elevation and the receiverand transmitter are fixed. It is thus necessary to provide a joint whichwill allow the motion of the antenna and yet present no discontinuityalong the joint between the antenna and the fixed receiver andtransmitter. Since the rotary jointmust present no discontinuity withcontinuous rotation, the parts of the joint must appear to the fieldpatterns as circularly symmetrical. Likewise, it is required that boththe transmit signal and receive signal travel along the same rotaryjoint. It may also be required that a third signal, which may be atracking signal, travel along the same rotary joint. This dual channelor three channel rotary joint must be one in which there is a minimum ofcross coupling between the transmit and receive signals and/or trackingsignal traveling along this joint. This leads to the problem of theselection of the type of modes to be excited on either side of the jointand the manner in which there modes may most efficiently be excited sothat the overall unit is relatively compact and yet, maintain isolationbetween the signals.

Rotary joints of the type using TEM modes of propagation with coaxiallines are often used because they meet the proper symmetry for a rotaryjoint. This type of rotary joint using the coaxial lines is notpractical however for many high average power applications because ofthe excessive heating of the inner conductor.

Systems for providing rotary joints using the TE mode are known. Thistype of rotary joint becomes impractical in applications requiringlimited space because of the large size type of TE mode exciter designknown in the present state of the art.

Another approach for providing a dual channel rotary joint is that ofusing counter rotating circularly polarized TE modes for transmit andreceive signals. This approach has a disadvantage in that the two fieldconfigurations are in the same basic mode types and any symmetricalimpedance mismatch in the rotary junction section will result in crosscoupling between the transmit channel and receive signals.

It is therefore an object of the present invention to provide a novel TMmode exciter.

It is a further object of the present invention to provide an improvedmultimode exciter which is relatively compact and provides a minimum ofcross coupling between signal channels.

Briefly, this and other objects of the present invention areaccomplished by a pair of elongated apertures in a broad wall of arectangular waveguide with a circular waveguide joined symmetrically tothe apertured region which, in response to electromagnetic waves in therectangular waveguide excite only the TM mode in the circular waveguide.The pair of elongated apertures are located on the opposite sides of thecenter line of the waveguide and extend in the direction of thelengthwise axis of the rectangular waveguide.

This invention will be better understood by reference to theaccompanying drawings in which:

FIG. 1 is a perspective view of a TM. mode exciter and a multimodeexciter using same,

FIG. 2 is a block diagram of a dual channel rotary joint system, and

FIG. 3 is a sketch of the rotary joint section associated with the dualchannel rotary joint system.

Referring to FIG. 1, there is illustrated the geometry of a multimodeexciter l0 capable of exciting the TM mode and two orthogonal TE modesin circular waveguide 31. The TM mode :is excited in circular waveguide31 by the circular waveguide 31 being joined to a properly aperturedbroad wall 28 of rectangular waveguide 29. The apertures 25 and 27 inthe broad wall 28 are narrow, elongated and are equidistant from and onthe opposite sides of the center line of the waveguide 29. The elongatedlength of the apertures 25 and 27 is in the direction of propagation ofelectromagnetic waves in the rectangular waveguide 29. The circularwaveguide 31 is joined to the broad wall 28 such that the center line ofthe circular waveguide 31 extends perpendicular and through the centerline of the rectangular waveguide 29 and midway between the apertures 25and 27. One end 33 of waveguide 29 is shorted. This short-circuit may beprovided by a conductive panel across the aperture of waveguide 29. Atthe opposite end 35 of waveguide 29 electromagnetic waves are coupled toor from the rectangular waveguide 29.

The electromagnetic wave signal energy travels along the rectangularwaveguide 29 from end 35 in the dominant TE mode. The end 33 of therectangular waveguide 29 is shorted at a distance from the elongatedapertures 25 and 27 such as to provide the best impedance match at therectangular waveguide input end 35. The signal energy traveling alongthe rectangular waveguide 29 in the TE mode excites electromagneticwaves in the TM mode in circular waveguide 31. The apertures 25 and 27and the circular waveguide 31 are arranged and dimensioned to providemaximum energy transfer from the TE mode in the rectangular waveguide 29to the TM mode in the circular waveguide 31.

By the placement of the two such elongated apertures or slots 25 and 27in the broad wall 28 equidistant from and on opposite sides of thecenter line of the rectangular waveguide 29, the aperture currents areequal in magnitude and flow in opposite directions. In this manner, theTE mode in the circular waveguide 31 is suppressed at all frequenciesand only the next two higherorder modes, TM and TE are excited. By theproper selection of the diameter of the circular waveguide 31, theundesired TE mode is below cutoff, thus leaving only the TM mode.Because of the reciprocity nature of such microwave devices,electromagnetic wave energy in the TM mode traveling along the circularwaveguide 31 toward elongated apertures 25 and 27 are coupled out ofrectangular waveguide 29 in the TE mode.

Referring to FIG. 1, a rectangular waveguide 37 is joined at rightangles to the circular wave guide 31 to excite in the circular waveguide31 electromagnetic wave energy in the linearly polarized TE mode. Inputand output electromagnetic wave energy traveling along waveguide 37 isapplied to or taken from the open end 41 of the waveguide 37. Couplingof this energy from rectangular waveguide 37 to circular waveguide 31 isprovided by three elongated parallel apertures or slots 43 in the wallof circular waveguide 31. The breadth of the broad wall of therectangular waveguide 37 extends along the lengthwise axis of thecircular waveguide 31. The elongated apertures 43 in the wall of thecircular waveguide 31 extend in the direction of the lengthwise axis ofthe circular waveguide 31 and parallel to the broad walls of waveguide37. Electromagnetic wave energy applied at terminal 41 of rectangularwaveguide 37 propagates in a dominant TE mode in the waveguide 37 towardthe apertures 43. At the rectangular to circular waveguide interface 39the linearly polarized TE mode electromagnetic waves are excited and thehigher order TE and TE mode waves are excited in the circular waveguide31. By the proper selection of the circular waveguide 31 diameter, thehigher order TE and TE mode waves are below cutoff and die out rapidlywithin a short distance from the junction of the two waveguides.

Referring to FIG. 1 the elongated-apertures 43 in the waveguide 31areoriented at right angles relative to the elongated apertures 25 and27 that are in rectangular waveguide 29. Because of this orthogonalrelationship between narrow elongated apertures the energy in thelinearly polarized TE mode excited at the apertures 43 sees only areflective radio frequency short and not the elongated apertures 25 and27. In accordance with the principles of reciprocity of excitercouplers, properly oriented electromagnetic wave energy in the linearlypolarized TE mode traveling along waveguide 31 excites in therectangular waveguide 37 wave energy in the TE mode. This properlyoriented TE mode wave energy is that wherein the electric field of thewave extends perpendicular to the broad walls of the waveguide 37.

By having a series of elongated narrow apertures 43 in the circularwaveguide 31 rather than a single relatively wider aperture andrectangular waveguide coupled thereabout, the impedance discontinuity tothe TM. mode propagating in the circular waveguide 31 is less. As afurther precaution to maintain the symmetry to the TM. mode, identicalthree parallel elongated apertures 45 are made in the circular waveguide31 directly opposite that of the three elongated apertures 43. A shortedsection of waveguide 47 is symmetrically joined to the circularwaveguide 31 at the region of the apertures 45 in the circular waveguide31. The breadth of the broad walls of the waveguide 47 extends in thedirection of the lengthwise axis of the circular waveguide. Since thewave energy in the TE mode in the circular waveguide 31 will be coupledthrough the elongated apertures 45, a further advantage is obtained inthat the length of the short circuited rectangular waveguide 47 can beemployed as an additional impedance matching element for the wave energyin the linearly polarized TE mode excited at the apertures 43.

Electromagnetic energy in a third mode isolated from the other two modescan be excited in the circular waveguide 31 by an elongated narrowaperture 51 in the broad wall 54 of rectangular waveguide 29 and a thirdrectangular waveguide 55 is symmetrically joined to this aperture 51.The broad wall 54 is opposite that of broad wall 28 of waveguide 29. Theaperture 51 extends along the center line of the waveguide 29 and iscentered with respect to an extension of the axis of the circularwaveguide 31 and is centered with respect to an extension of theapertures 25 and 27. The rectangular waveguide 55 is positioned suchthat the width of the broad wall is in the direction of the lengthwiseaxis of the waveguide 29.

When electromagnetic wave energy is coupled at end 56 of waveguide 55,the signal propagates in the TB mode toward the aperture 51. Since theaperture 51 is symmetrically placed relative to the apertures 25 and 27,the currents at the apertures 25 and 27 are equal in magnitude and flowin the same direction. Therefore wave energy in the TE mode at thewaveguide 55 excites in the circular waveguide 31 wave energy in thelinearly polarized TE mode. Due to the orthogonal relationship betweenthe elongated narrow apertures 25 and 27 in waveguide 29 and theelongated narrow apertures 43 in the waveguide 31, the polarization ofthe linearly polarized TE mode waves excited through apertures 25 and 27into circular waveguide 31 is orthogonal with respect to thepolarization of the TE mode signal waves excited through apertures 43.Consequently, wave energy excited through apertures 25 and 27 isisolated from the wave energy excited through apertures 43. Due to thereciprocal nature of these devices, properly oriented linearly polarizedTE mode wave energy traveling along circular waveguide 31 is transferredthrough apertures 25 and 27 and aperture 51 to rectangular waveguide 55in the TB mode without being coupled through aperture 43. This properlyoriented TE mode wave energy is that where the electric field of thewave is perpendicular to the elongated apertures 25 and 27.

The wave energy coupled through aperture 51 in waveguide 29 is isolatedfrom terminal 35 of waveguide 29 because the height h of the rectangularwaveguide 29 is insufficient to support an electromagnetic field of amode which would get excited in waveguide 29. The wave energy in the TEmode traveling along waveguide 29 is isolated from waveguide 55 becausethe centered placement of the narrow elongated aperture 51 makes it atthe lowest coupling point of the waveguide 29.

It is often desirable, particularly when the electromagnetic waves mustpropagate across a rotary joint, to transform the orthogonal linearlypolarized TE, mode waves excited at apertures 43 and those excited bythe combination of apertures 25, 27 and 51 into respective right andleft circularly polarized TE, mode signals. Referring to FIG. 1, ridges57 and 58 are placed on the opposite sides of the circular waveguide 31.These ridges 57 and 58 lie in a plane oriented at a 45 angle relative tothat of 'waveguide 37 and the lengthwise axis of apertures 25 and 27.The incoming linearly polarized TE mode waves excited at apertures 43and orthogonal apertures 25 and, 27 and applied toward ridges 57 and 58may each be resolved into two orthogonal components of equal amplitudeand phase, one which is aligned parallel to the plane of the ridges 57and 58 and the other perpendicular. The different loading caused by theridges 57 and 58 to these two orthogonal components results in anunequal phase velocity for these two components. By the proper design ofridges 57 and 58, as is well known in the state of the art, the twoorthogonal components emerging from the far end of the ridges 57 and 58which make up the polarizer have equal amplitudes with quadraturephasing or circular polarization.

Linearly polarized TE mode waves excited at the apertures 43 in thecircular waveguide 31 are converted to a circular polarized TE modewaves in a first direction. The orthogonal linearly polarized TE modewavesexcited at the apertures 25 and 27 are converted to circularpolarized TE mode waves rotating in a direction opposite the firstdirection.

Since ridges 57 and 58 are symmetrical relative to the center of thecircular waveguide 31, they do not cause any mode conversion between theTM mode and the TE, mode. Also the ridges may be designed to have arelatively small penetration, about one-eighth inch, for example, intothe circular waveguide so as to minimize the impedance discontinuity tothe TM mode waves. Because of this small penetration, the length of theridges to obtain quadrature phasing may be relatively long from abouteight to ten inches for receive signals in the 7250 to 7750 megahertzfrequency range. The ends 63 of the ridges are tapered to reduce theimpedance discontinuity presented by the ridges.

Referring to FIG. 2, there is illustrated a block diagram of a dualchannel rotary joint system 60. The system 60 is made up of a firstmultimode exciter and polarizer system 61 and a second multimode exciterand polarizer system 63 and a rotary joint section 66 coupledtherebetween. The multimode exciter and polarizer system .61 may be arotating structure coupled to a rotating antenna and the multimodeexciter and polarizer system 63 may be a stationary structure coupled toa stationary receiver and transmitter. The multimode exciter andpolarizer system 61 is made up of a TM mode exciter 62, a linearlypolarized TE mode exciter 64 and a polarizer section 65. The multimodeexciter and polarizer system 63 is made up of a TM mode exciter 62a, alinearly polarized TE mode exciter 640, a reflection absorbing load 67,and a polarizer section 650.

The TM mode exciters 62 and 62a are each constructed in accordance withFIG. 1 wherein dual elongated apertures are cut in the broad walls of afirst rectangular waveguide and a circular waveguide is joinedsymmetrically thereto. The linearly polarized TE mode exciters 64 and64a are each constructed like that in FIG. 1 wherein apertures similarto apertures 43 are located in the circular waveguide and a secondrectangular waveguide similar to rectangular waveguide 37 is joinedsymmetrically thereto. The polarizers 65 and 65a are each provided by apair of ridges similar to 57 and 58 of FIG. 1 in the circular waveguidewhich ridges lie in a plane oriented at a 45 angle with respect to thatof the second rectangular waveguide.

In thev dual channel rotary joint system, the two circular waveguidesections associated with the exciters are each positioned in alignedrelation to the other with the free ends of the circular waveguidesection spaced from each other by a rotary joint section 66. Thereflection absorbing load 67 of multimode exciter and polarizer system63 is provided by a third waveguide and aperture centered in the broadwall of the first waveguide opposite that having dual slots therein.This configuration is like that illustrated in FIG. 1 wherein waveguide55 is junctured with waveguide 29 having aperture 51 therein. This thirdwaveguide which forms a part of load 67 has microwave signal-absorbingmaterial therein. As described previously in connection with FIG. 1,neither wave energy in the TM mode nor the wave energy in the properlyoriented linear TE mode associated with the second waveguide are coupledto load 67. Any linear TE mode signals orthogonal to the above properlyoriented linear TE mode signals, such as that caused by reflections fromthe rotary joint section 66, would be coupled to load 67. Thisabsorption of the small reflected wave reduces the output signalvariation as a function of angular rotation of the rotating multimodeexciter and polarizer system 61 relative to the fixed multimode exciterand polarizer system 63. This absorption of the reflected waves furtherreduces the voltage standing wave ratio of the linearly polarized TEmode channel associated with exciters '64 and 64a with only a smallpower loss for reasonably small impedance mismatches.

Referring to FIG. 3, there is illustrated in cross section the rotaryjoint section 66 which is made up of two circular waveguide sections 71,73. Circular waveguide sections 71 and 73 may be an extension of thecircular waveguides which are in aligned relation to each other andwhich are each represented in FIG. 1 as circular waveguide 31. Waveguidesection 71 has a reduced wall thickness and cross section at one endnear the center of the rotary joint 66 such as to fit coaxially withinand along the waveguide section 73 having an increased aperture with agap 75 between the two waveguide sections 71 and 73. The length of theoverlap of the two sections 71 and 73 from the exterior gap point 77 tothe interior gap point 79 is approximately one-quarter wavelength longat the mean operating frequency of the dual channel rotary joint systembetween the transmit and receive frequency band. By making the gapbetween the two circular waveguide sections 71 and 73 small, a lowcharacteristic impedance coaxial line one-quarter wavelength long at themean frequency stated above between gap points 77 and 79.

A pair of one-quarter wave choke flanges 81 and 83 are mounted coaxialwith and to the respective circular waveguide sections 71 and 73. Theflanges 81 and 83 are mounted close to each other with a gap 85 betweenthe flanges. The gap 85 is arranged to be aligned with the exterior gappoint 77. The flanges 81 and 83 are shorted at ends 87 and 89 with therespective waveguide sections 71 and 73 one-quarter wavelength at themean frequency of the dual channel system from the gaps 75 and 85. Bythe placement of the gap 85 in the middle between two flanges, bothflanges being identical, two one-quarter wavelength chokes are providedin series. Since the two circular waveguide sections 71 and 73 provide arelatively low impedance transmission line and since the gap 85 is aquarter wavelength from the short circuited points 87 and 89, the gap 85presents a relatively high impedance relative to that of the gap 75.Because there are two quarter wavelength flanges in series, the gap 85presents an even higher impedance relative to that of the overlappedcircular waveguide sections 71 and 73. This se ries impedance furtherincreases the overall bandwidth of the system.

The rotary joint must operate with a minimum of discontinuity for bothelectromagnetic waves in the TE mode and in the TM mode of propagation.The relative phase velocity in the quarter wavelength coaxia] line madeup of sections 71 and 73 spaced by the gap 75 is unity for waves in theTM mode. Electromagnetic waves in the TM mode have a constant phasevoltage around the gap 79. Electromagnetic wave energy in a circularlypolarized TE, mode in the circular waveguide generates aprogressively-phased voltage around the gap 79. The one-quarterwavelength coaxial line formed by the overlap of the waveguide sections71 and 73 becomes a coaxial waveguide supporting waves in the TB mode.Because ofthe close spacing between the inner and outer conductors ofthis coaxial onequarter wavelength transmission line, the relative phasevelocity for the waves in the TE mode is only a few percent greater thanthat of the waves in the TM mode. The overall operation of the dualchannel rotary joint system 60 can be understood by reference to FIG. 2.Transmit signal energy from a fixed transmitter is coupled to TM. modeexciter 62a of multimode exciter and polarizer 63. This transmit signalenergy excites waves in the TM mode in the circular waveguide system ofmultimode exciter and polarizer 63. The transmit signal energy in the TMmode in the circular waveguide propagates through the polarizer 65a,along sections 71 and 73 of the rotary joint section 66 throughpolarizer section 65 to the TM mode exciter 62 in the rotatablemultimode exciter and polarizer system 61. At the TM. mode exciter 62,transmit signal energy in the TM. mode is converted to energy in the TEmode in a rectangular waveguide. Received signal energy from a rotatingantenna is coupled to the linearly polarized TE mode exciter 64 ofrotatable multimode exciter and polarizer system 61. At the exciter 64,the received signal energy in the TE mode traveling along a rectangularwaveguide is converted to wave energy in the linearly polarized TE modein the circular waveguide. The linearly polarized TB mode wave energy iscircular polarized at the polarizer section 65. This circularlypolarized TE, mode signal energy is coupled along the aligned circularsections 71 and 73 of rotary joint 66 to the polarizer 65a of themultimode exciter and polarizer system 63. At the polarizer 65a, thereceived energy in the circularly polarized TE mode is converted toreceived energy in linearly polarized TE mode and is coupled to thelinearly polarized TE mode exciter 64a. At the exciter section 64a, thereceived signal energy in the linear TE mode is converted to receivedsignal energy in the TB mode in rectangular waveguide through aperturessimilar to that of apertures 43 in FIG. 1. This received signal energyis then coupled to a fixed receiver terminal. As mentioned previously,linearly polarized TE mode wave energy reflected from the rotary jointsection 66 is coupled through the dual aperture in the broad wall andthe single aperture on the opposite side of the broad wall of thewaveguide.

These reflected signals from the rotary joint are then coupled to thewaveguide section 67 which includes an absorbing load.

In the operation of the system as described herein with a receiveroperating between 7250 to 7750 MH and a transmitter operating between7900 and 8400 MH the TM, mode exciter had the following dimensions:

Rectangular waveguide 29 is WR 137 waveguide (1.372 inch height X 0.622width inside dimensions),

Apertures 25 and 27: width 0.11 wavelengths and length 0.566 at the meantransmitter operating frequency, apertures 25 and 27 offset from centerline of rectangular waveguide 29 0.263 wavelengths at the meantransmitter operating frequency or about 0.378 inch from the centerline, and apertures each about 0.15 inch width and 0.770 inch long,

Waveguide 29 is terminated in a short circuit about 1.1 inches from thecenter of the apertures 25 and 27,

Circular waveguide 31: diameter 0.9 wavelengths at the mean operatingfrequency between transmitter and receiver frequency or for thearrangement described about 1.3 inch diameter,

Rectangular waveguides 37 and 47 (Wk-112) for mean receiver frequencydescribed about 0.497 inch height X 1.122 inch width inside dimension,

Apertures 43 and 45: width 0.116 inch, length 1.10

inch,

The ridges were about inch in depth and about 10.962 inches long,

The waveguide 55 is WR-9O waveguide, and

Aperture 51: 0.062 inch wide X 0.770 inch length,

In the operation of one described above, each mode exciter had a VSWR ofless than 1.2 to 1 over a band of at least 7 percent. When two suchexciters were placed together and joined by means of the rotary jointdescribed, the output signal variation as a function of angular rotationwas found to be less than 0.05 db over both transmit and receivefrequency bands.

While the above description was that of a dual channel rotary jointwaveguide system, this multimode exciter and polarizer system as shownin FIG. 1 is capable of providing a three channel waveguide system. Thefirst or the transmit channel waveguide system is that provided by theexcitation of waves in the TM mode and waves through the rotary jointsection as described above, including the two TM mode exciters 62 and62a of FIG. 2. The second or receive channel is that provided by theexcitation of waves in the TB mode, the circular polarization of thesewaves and coupling of these circular polarized TE mode waves through,the rotary joint section 66 as described in connection with exciters 64and 64a and polarizer 65 and 650. A third channel waveguide system canbe had by excitation of waves in a second linearly polarized TE modeorthogonal to the first by the combination of the dual apertures on oneof the broad walls of the rectangular waveguide and the single apertureon the opposite broad wall of that waveguide as illustrated by apertures25, 27 and 51 in waveguide 29 of FIG. 1. Since the linear TE mode wavesassociated with the second channel are orthogonal to the third, thechannels are isolated from each other and when passed throughthepolarizer they have opposite directions of circular polarization. Byremoving the absorption material in load 67 of FIG. 2 and using theassociated waveguide as a part of the third channel system and by addinga similar coupling structure 67a (indicated by dashed lines in FIG. 2)at the multimode exciter and polarizer system 61, a third channel systemmay be provided. In the third channel system a signal is excited in thelinear TE mode at exciter 67. This signal is circular polarized at 65aand is coupled through the rotary joint section 66 to polarizer 65. Atpolarizer 65 this third channel signal is converted from circulartolinear polarization and is coupled through exciter 67a to a thirdchannel terminal 68 in the rotating multimode exciter and polarizersystem 61.

What is claimed is:

l. A TM exciter comprising:

a rectangular ,waveguide structure havinga pair of broad walls extendingbetween a pair. of narrow walls for propagating electromagneticwaves,said rectangular waveguide structure having a pair of apertures disposedon one of the broadwalls of said rectangular waveguide structure withsaid apertures being located on opposite sides of that center line ofsaid one broad wall between the two narrow walls, and a circularwaveguide structure joined normal to said one broad wall of saidrectangular waveguide structure with the circular waveguide structurebeing symmetrically disposed relative to said apertures to provide, whenan electromagnetic wave is propagated toward the junction through therectangular waveguide in the TE mode, maximum energy transfer into thecircular waveguide structure in the TM mode with energy in TE mode beingsuppressed.

2. The combination as claimed in claim 1 wherein said apertures areelongated and extend along a portion of the length of said rectangularwaveguide.

3. The combination'as claimed in claim 2 wherein said elongatedapertures are equidistant from the center line of the rectangularwaveguide.

4. A multimode exciter comprising:

a first rectangular waveguide structure for propagation ofelectromagnetic waves,

said first rectangular waveguide structure having a pair of broad wallsextending between a pair of narrow walls, said first rectangularwaveguide structure having a pair of elongated apertures disposed in oneof the broad walls of said first rectangular waveguide structure withthe apertures being symmetrically disposed on opposite sides of thatcenter line of said one broad wall between the two narrow walls,circular waveguide structure disposed normal to said one broad wall ofsaid first rectangular waveguide structure with the circular waveguidestructure symmetrically disposed relative to the pair of elongatedapertures to provide, when electromagnetic wave energy is propagatedtoward the junction along the first rectangular waveguide in the TEmode, maximum energy transfer into the circular waveguide structure inthe TM mode with energy in the TE mode being suppressed, said circularwaveguide structure having at least one elongated aperture extending,along the lengthwise axis of the circular waveguide wall,

a second rectangular waveguide structure joined normal to said circularwaveguide structure with the second rectangular waveguide structurebeing disposed relative to said aperture in the wall of said circularwaveguide structure to provide, when electromagnetic wave energy ispropagated toward the junction of the second rectangular waveguidestructure and said circular waveguide structure through the secondrectangular waveguide structure in the TE mode, maximum energy transferinto the circular waveguide structure in the TE mode.

5. The combination as claimed in claim 4 wherein the broad wall of saidsecond rectangular waveguide structure is parallel to the lengthwiseaxis of said circular waveguide structure.

6. The combination as claimedin claim 5 wherein a pair of ridges arelocated within said circular waveguide structure.

7. The combination as claimed in claim 6 wherein said pair of ridges arealigned on opposite sides of said circular waveguide structure such thatthe angle formed by the intersection of a plane through the crosssectional center of the ridges and a plane through the cross sectionalcenter of the second rectangular waveguide is equal to about 45 degrees.

8. The combination as claimed in claim 7 wherein a plurality of narrowelongated rectangular apertures are located in the wall of said circularwaveguide structure with said second rectangular waveguide structuremounted perpendicular to said circular waveguide structure about theregion of said apertures.

9. The combination as claimed in claim 8 wherein said narrow elongatedapertures in said circular waveguide structure have their lengthwiseaxis parallel to the broad walls of said second rectangular waveguide.

10. The combination as claimed in claim 9 including a third rectangularwaveguide structure perpendicular to and aligned with on opposite sideof said circular waveguide structure relative to said second rectangularwaveguide structure, said third rectangular waveguide structure beingterminated in a short circuit with the broad wall along the lengthwiseaxis of said circular waveguide, said circular waveguide having aplurality of elongated apertures at the junction region of said thirdrectangular waveguide to said circular waveguide structure.

11. A multimode exciter comprising:

a first rectangular waveguide structure for propagation ofelectromagnetic waves,

said first rectangular waveguide structure having a pair of slot-likeapertures disposed in one of the broad walls of said first rectangularwaveguide structure with the apertures being symmetrically disposed onopposite sides of the center line of said waveguide, circular waveguidestructure disposed normal to said one broad wall of said firstrectangular waveguide structure with the circular waveguidesymmetrically disposed relative to said pair of apertures to provide,when electromagnetic energy is propagated toward the junction along thefirst rectangular waveguide in the TE mode, maximum energy into thecircular waveguide in the TM mode,

said first rectangular waveguide having a single slotlike aperturecentered along a broad wall opposite said first-mentioned broad wall,

a second rectangular waveguide structure disposed normal to saidopposite broad wall of said first rectangular waveguide structure toprovide, when electromagnetic energy is propagated toward the singleaperture along said second rectangular waveguide in the TE mode, maximumenergy transfer into the circular waveguide in the linear TE mode.

12, A waveguide system including a rotary joint for the coupling ofsignals within at least two frequency bands through said rotary jointcomprising:

a first circular waveguide structure,

a second circular waveguide structure spaced from said first circularwaveguide structure by said rotary joint,

said first and second circular waveguide structures each having firstand second ports,

means responsive to signals within a first of said frequency bands atthe first port of said first circular waveguide structure for excitingsaid signals within a first of said frequency bands in the TM mode alongsaid first circular waveguide structure,

means for coupling said signals within said first frequency band in theTM mode from the first circular waveguide structure through the rotaryjoint to said second circular waveguide structure,

means responsive to said signal within said first frequency band signalsin the TM mode at said LII second circular waveguide structure for thecoupling of said signals within said first frequency band in the TM modeout of the first port of said second circular waveguide structure, meansresponsive to signals within a second of said frequency bands at thesecond port of said second circular waveguide structure for excitingsaid signal within said second frequency band in the linearly polarizedTE mode along said second circular waveguide structure, means along saidsecond circular waveguide structure responsive to said second frequencyband signals in said linearly polarized TE mode for transforming saidsecond frequency band signals to circularly polarized TE mode signals,means for coupling said second frequency band signals in the circularlypolarized TE mode from said second circular waveguide structure throughsaid rotary joint to said first circular waveguide structure, means atsaid first circular waveguide structure responsive to said secondfrequency band signals in said circularly polarized TE mode fortransforming said second frequency band circularly polarized TE modesignals to second frequency band linearly polarized TE mode signals, and

means responsive to said second frequency, linearly polarized TE modesignals at said first circular waveguide structure for the coupling ofsaid signal in said second frequency band out of the second port of saidfirst circular waveguide structure.

13. A waveguide system including a rotary joint for the coupling ofsignals within at least two frequency bands through said rotary jointcomprising:

a first circular waveguide structure,

a second circular waveguide structure,

said first and second circular waveguide structures having a common axiscoupled to each other end to end through said rotary joint, each of thefree ends of each of said circular waveguide structures being joined totwo mutually perpendicular rectangular waveguides, said rectangularwaveguides having their axes perpendicular to the axis of said circularwaveguide structures,

the first rectangular waveguide of said two rectangular waveguides andsaid first circular waveguide structure in coupling relation with eachother so that signals within a first of said frequency bands along saidfirst rectangular waveguide provide maximum energy transference into thecircular waveguide structure in the TM, mode and propagation of saidsignals within the first frequency band in the TM mode occurs, whereuponsaid TM mode signals propagate along said first circular waveguidestructure through said rotary joint to said second circular waveguidestructure, the second rectangular waveguide of said two rectangularwaveguides joined to said second circular waveguide structurecommunicating with said second circular waveguide structure so as toprovide coupling of said signals within the first frequency band in theTM mode out of said second rectangular waveguide,

a third rectangular waveguide structure joined to said second circularwaveguide structure communicating with said second circular waveguidestructure to provide maximum energy transfer of said signals within thesecond frequency band of said two frequency bands to said secondcircular waveguide structure in the linearly polarized TE mode alongsaid second circular waveguide structure,

means along said second circular waveguide struc ture responsive to thesecond frequency band signals in said linearly polarized TE mode fortransforming the second frequency band signals to circularly polarizedTE mode signals whereupon said circularly polarized TE mode signals arecoupled along said second circular waveguide structure and said rotaryjoint to said first circular waveguide structure,

means at said first circular waveguide structure responsive to thesecond frequency hand signals in said circularly polarized TE mode fortransforming said second frequency band signals to linearly polarizedTE, mode signals, and

a fourth rectangular waveguide structure joined to said first circularwaveguide structure communicating with said first circular waveguidestructure so that said TE mode signals within the second frequency bandare coupled out of said second rectangular waveguide structure joined tosaid first circular waveguide structure.

14. In combination:

a first rectangular waveguide having a pair of apertures in one broadwall equally disposed on opposite sides of said one broad wall of saidfirst rectangular waveguide,

a first and a second circular waveguide spaced from and communicatingwith each other by a rotary joint, said first circular waveguide joinednormal to 1 said first rectangular waveguide with said first circularwaveguide disposed relative to said apertures to provide maximumtransfer of energy through the first rectangular waveguide in the TEmode to said first circular waveguide in the TM mode, said firstcircular waveguide having at least one'aperture along the waveguidingwall,

a second rectangular waveguide positioned normal to said first circularwaveguide and about said aperture in the wall of said first circularwaveguide so as to be in coupling relation to said first circularwaveguide for the coupling; of energy in the linearly polarized TE modefrom said first circular waveguide to said second rectangular waveguide,

said second circular waveguide having an aperture along the waveguidingwall,

a third rectangular waveguide having a pair of apertures in one broadwall equally disposed on opposite sides of said one broad wall of saidthird rectangular waveguide,

said second circular waveguide mounted normal to said third rectangularwaveguide with the second circular waveguide disposed relative to saidapertures to provide maximum transfer of energy from the second circularwaveguide in the TM mode to the third rectangular waveguide in the TEmode a fourthrectangular waveguide joined normal to said second circularwaveguide and about said aperture in the waveguiding wall of said secondcircular waveguide so as to be in coupling relation to said secondcircular waveguide to provide maximum energy transfer through the fourthrectangular waveguide in the TB mode into the second circular waveguidein the linearly polarized TE mode, said first and said second circularwaveguides each having ridges within said respective first and secondcircular waveguides, said ridges in said second circular waveguide inresponse to said linearly polarized TE mode signals providing circularlypolarized TE mode signals at said rotary joint, said ridges within saidfirst circular waveguide responsive to said circularly polarized TE modesignals coupled to said first circular waveguide through said rotaryjoint converting said circularly polarized TE mode signals to linearlypolarized TE mode signals.

v I! i

1. A TM01 exciter comprising: a rectangular waveguide structure having apair of broad walls extending between a pair of narrow walls forpropagating electromagnetic waves, said rectangular waveguide structurehaving a pair of apertures disposed on one of the broad walls of saidrectangular waveguide structure with said apertures being located onopposite sides of that center line of said one broad wall between thetwo narrow walls, and a circular waveguide structure joined normal tosaid one broad wall of said rectangular waveguide structure with thecircular waveguide structure being symmetrically disposed relative tosaid apertures to provide, when an electromagnetic wave is propagatedtoward the junction through the rectangular waveguide in the TE10 mode,maximum energy transfer into the circular waveguide structure in theTM01 mode with energy in TE11 mode being suppressed.
 1. A TM01 excitercomprising: a rectangular waveguide structure having a pair of broadwalls extending between a pair of narrow walls for propagatingelectromagnetic waves, said rectangular waveguide structure having apair of apertures disposed on one of the broad walls of said rectangularwaveguide structure with said apertures being located on opposite sidesof that center line of said one broad wall between the two narrow walls,and a circular waveguide structure joined normal to said one broad wallof said rectangular waveguide structure with the circular waveguidestructure being symmetrically disposed relative to said apertures toprovide, when an electromagnetic wave is propagated toward the junctionthrough the rectangular waveguide in the TE10 mode, maximum energytransfer into the circular waveguide structure in the TM01 mode withenergy in TE11 mode being suppressed.
 2. The combination as claimed inclaim 1 wherein said apertures are elongated and extend along a portionof the length of said rectangular waveguide.
 3. The combination asclaimed in claim 2 wherein said elongated apertures are equidistant fromthe center line of the rectangular waveguide.
 4. A multimode excitercomprising: a first rectangular waveguide structure for propagation ofelectromagnetic waves, said first rectangular waveguide structure havinga pair of broad walls extending between a pair of narrow walls, saidfirst rectangular waveguide structure having a pair of elongatedapertures disposed in one of the broad walls of said first rectangularwaveguide structure with the apertures being symmetrically disposed onopposite sides of that center line of said one broad wall between thetwo narrow walls, a circular waveguide structure disposed normal to saidone broad wall of said first rectangular waveguide structure with thecircular waveguide structure symmetrically disposed relative to the pairof elongated apertures to provide, when electromagnetic wave energy ispropagated toward the junction along the first rectangular waveguide inthe TE10 mode, maximum energy transfer into the circular waveguidestructure in the TM01 mode with energy in the TE11 mode beingsuppressed, said circular waveguide structure having at least oneelongated aperture extending along the lengthwise axis of the circularwaveguide wall, a second rectangular waveguide structure joined normalto said circular waveguide structure with the second rectangularwaveguide structure being disposed relative to said aperture in the wallof said circular waveguide structure to provide, when electromagneticwave energy is propagated toward the junction of the second rectangularwaveguide structure and said circular waveguide structure through thesecond rectangular waveguide structure in the TE10 mode, maximum energytransfer into the circular waveguide structure in the TE11 mode.
 5. Thecombination as claimed in claim 4 wherein the broad wall of said secondrectangular waveguide structure is parallel to the lengthwise axis ofsaid circular waveguide structure.
 6. The combination as claimed inclaim 5 wherein a pair of ridges are located within said circularwaveguide structure.
 7. The combination as claimed in claim 6 whereinsaid pair of ridges are aligned on opposite sides of said circularwaveguide structure such that the angle formed by the intersection of aplane through the cross sectional center of the ridges and a planethrough the cross sectional center of the second rectangular waveguideis equal to about 45 degrees.
 8. The combination as claimed in claim 7wherein a plurality of narrow elongated rectangular apertures arelocated in the wall of said circular waveguide structure with saidsecond rectangular waveguiDe structure mounted perpendicular to saidcircular waveguide structure about the region of said apertures.
 9. Thecombination as claimed in claim 8 wherein said narrow elongatedapertures in said circular waveguide structure have their lengthwiseaxis parallel to the broad walls of said second rectangular waveguide.10. The combination as claimed in claim 9 including a third rectangularwaveguide structure perpendicular to and aligned with on opposite sideof said circular waveguide structure relative to said second rectangularwaveguide structure, said third rectangular waveguide structure beingterminated in a short circuit with the broad wall along the lengthwiseaxis of said circular waveguide, said circular waveguide having aplurality of elongated apertures at the junction region of said thirdrectangular waveguide to said circular waveguide structure.
 11. Amultimode exciter comprising: a first rectangular waveguide structurefor propagation of electromagnetic waves, said first rectangularwaveguide structure having a pair of slot-like apertures disposed in oneof the broad walls of said first rectangular waveguide structure withthe apertures being symmetrically disposed on opposite sides of thecenter line of said waveguide, a circular waveguide structure disposednormal to said one broad wall of said first rectangular waveguidestructure with the circular waveguide symmetrically disposed relative tosaid pair of apertures to provide, when electromagnetic energy ispropagated toward the junction along the first rectangular waveguide inthe TE10 mode, maximum energy into the circular waveguide in the TM01mode, said first rectangular waveguide having a single slot-likeaperture centered along a broad wall opposite said first-mentioned broadwall, a second rectangular waveguide structure disposed normal to saidopposite broad wall of said first rectangular waveguide structure toprovide, when electromagnetic energy is propagated toward the singleaperture along said second rectangular waveguide in the TE10 mode,maximum energy transfer into the circular waveguide in the linear TE11mode.
 12. A waveguide system including a rotary joint for the couplingof signals within at least two frequency bands through said rotary jointcomprising: a first circular waveguide structure, a second circularwaveguide structure spaced from said first circular waveguide structureby said rotary joint, said first and second circular waveguidestructures each having first and second ports, means responsive tosignals within a first of said frequency bands at the first port of saidfirst circular waveguide structure for exciting said signals within afirst of said frequency bands in the TM01 mode along said first circularwaveguide structure, means for coupling said signals within said firstfrequency band in the TM01 mode from the first circular waveguidestructure through the rotary joint to said second circular waveguidestructure, means responsive to said signal within said first frequencyband signals in the TM01 mode at said second circular waveguidestructure for the coupling of said signals within said first frequencyband in the TM01 mode out of the first port of said second circularwaveguide structure, means responsive to signals within a second of saidfrequency bands at the second port of said second circular waveguidestructure for exciting said signal within said second frequency band inthe linearly polarized TE11 mode along said second circular waveguidestructure, means along said second circular waveguide structureresponsive to said second frequency band signals in said linearlypolarized TE11 mode for transforming said second frequency band signalsto circularly polarized TE11 mode signals, means for coupling saidsecond frequency band signals in the circularly polarized TE11 mode fromsaid second circular waveguide structure through said rotary joint tosaid first circular waveguide structure, means at said first circularwaveguide structure responsive to said second frequency band signals insaid circularly polarized TE11 mode for transforming said secondfrequency band circularly polarized TE11 mode signals to secondfrequency band linearly polarized TE11 mode signals, and meansresponsive to said second frequency, linearly polarized TE11 modesignals at said first circular waveguide structure for the coupling ofsaid signal in said second frequency band out of the second port of saidfirst circular waveguide structure.
 13. A waveguide system including arotary joint for the coupling of signals within at least two frequencybands through said rotary joint comprising: a first circular waveguidestructure, a second circular waveguide structure, said first and secondcircular waveguide structures having a common axis coupled to each otherend to end through said rotary joint, each of the free ends of each ofsaid circular waveguide structures being joined to two mutuallyperpendicular rectangular waveguides, said rectangular waveguides havingtheir axes perpendicular to the axis of said circular waveguidestructures, the first rectangular waveguide of said two rectangularwaveguides and said first circular waveguide structure in couplingrelation with each other so that signals within a first of saidfrequency bands along said first rectangular waveguide provide maximumenergy transference into the circular waveguide structure in the TM01mode and propagation of said signals within the first frequency band inthe TM01 mode occurs, whereupon said TM01 mode signals propagate alongsaid first circular waveguide structure through said rotary joint tosaid second circular waveguide structure, the second rectangularwaveguide of said two rectangular waveguides joined to said secondcircular waveguide structure communicating with said second circularwaveguide structure so as to provide coupling of said signals within thefirst frequency band in the TM01 mode out of said second rectangularwaveguide, a third rectangular waveguide structure joined to said secondcircular waveguide structure communicating with said second circularwaveguide structure to provide maximum energy transfer of said signalswithin the second frequency band of said two frequency bands to saidsecond circular waveguide structure in the linearly polarized TE11 modealong said second circular waveguide structure, means along said secondcircular waveguide structure responsive to the second frequency bandsignals in said linearly polarized TE11 mode for transforming the secondfrequency band signals to circularly polarized TE11 mode signalswhereupon said circularly polarized TE11 mode signals are coupled alongsaid second circular waveguide structure and said rotary joint to saidfirst circular waveguide structure, means at said first circularwaveguide structure responsive to the second frequency band signals insaid circularly polarized TE11 mode for transforming said secondfrequency band signals to linearly polarized TE11 mode signals, and afourth rectangular waveguide structure joined to said first circularwaveguide structure communicating with said first circular waveguidestructure so that said TE11 mode signals within the second frequencyband are coupled out of said second rectangular waveguide structurejoined to said first circular waveguide structure.